There has been lots of discussion of a mission to Europa in the excellent thread on the Juno mission. I thought that since a Europa mission seems to be once again becoming a possibility, it deserved its own thread for news, updates and discussion. I thought I'd kick things off with a summary of past efforts on a Europa mission, and on where things stand now. If I make a mistake, please correct me!

In the course of its prime and extended missions, Galileo found evidence of liquid water under the icy surface of the planet. Planning began on a Europa Orbiter mission, with a projected arrival date of 2008, to confirm the presence of the ocean, characterize the thickness of the icy crust and identify places for a future landing. One thing to note about these earlier plans: they included a direct trajectory to Jupiter, presumably to minimize mission duration and qualms about RTGs re-entering Earth atmosphere after some (highly unlikely) targeting mishap. But NASA lacked a nice category of missions to place the Europa Orbiter in. Eventually it got lumped together with Pluto Express and Solar Probe in a Outer Solar System program labelled "Fire and Ice", a term which also got applied to the Galileo Europa Mission extension. Without a solid program to support it, (like Mars Exploration, Great Observatories, or Discovery) the mission looked like an orphan.

As Bruce Moomaw has well documented, attempts to kill off the Pluto mission led to a tug of war between NASA, the planetary scientists and the public, resulting in Congressional directives to NASA. Pluto Express became the Pluto/Kuiper Belt Explorer and then New Horizons and New Frontiers 1. (New Frontiers 2 is of course Juno.) But the cost for the Europa mission continued to rise, and the launch date recede, as the difficulty of radiation shielding and the large delta-v requirements hit home, and the mission's public profile fell. The launch date moved to 2010 and the costs moved over a $1b. Then along came Sean O'Keefe and JIMO, a justification for the Prometheus program through developing nuclear electric propulsion, not with RTGs, but with an in-space fission reactor. Launch got moved to 2011, then 2012, while the cost went even further through the roof.

With the arrival of Mike Griffin, JIMO was cancelled. As Griffin said to Congress, "It was not a mission, in my judgment, that was well-formed." But interest in a Europa mission remained and remains strong. In 2003 the National Academy of Science's Decadal Survey flatly stated that a Europa Orbiter was the top priority for the next Large scale (aka Flagship) mission. (See page 196 of the report.) NASA's current Solar System Exploration Roadmap reaffirmed a Europa orbiter as the next flagship mission. The question as always is money. As Administrator Griffin said, "The Science Mission directorate wants to do a Europa mission, the National Academy of Sciences wants to do a Europa mission, I want to do a Europa mission. When we can afford it in the budget, we'll do it."

Evidence of that support beyond rhetoric and reports trickled out with a letter from Andy Danzler, NASA's Solar System chief, to the Outer Planets Assessment Group (OPAG). He reported that he had "funded a team to take a quick look at the boundary conditions of a mission to Europa, that is, how much power, mass, travel time, etc. for various realistic scenarios. For planning purposes, this group is looking at launch dates in the 2012-2015 range, although the later dates are more likely in terms of funding." For funding details however, we have to wait for the FY 2007 budget.

OK, now the good stuff.

The latest meeting of OPAG included reports on a Reference Design for the mission. A kind of first draft which establishes a baseline which can be tweaked and modified to extract the best science return.There are many things to like about this draft design: * The mission is now permitted to use Earth flybys, and uses a proven trajectory, the same as used by Galileo (Venus-Earth-Earth Gravity Assist). This allows a BIG increase in the available mass. * The orbiter uses RTGs, but not super advanced ones that require further years of development. * The orbiter is similar to Cassini in appearance, with 2 engines, a cylindrical tank structure, RTGs at the base, the magetometer boom at the top, and space for a lander bolted to side. The similarities may make it easier to convince Congress that this is something NASA knows how to do. The most obvious configuration change is with science payload and HGA having switched places, and the addition of a radar array. And there looks like a camera the size of MRO's HiRISE! * The mission is definitely Flagship in scope with a launch mass of over 7000 kg on a heavy lift launch vehicle. For comparison Cassini was 5712 kg at launch on a Titan IV, and Galileo was 2223 kg when launched using the Shuttle and an Inertial Upper Stage. * There is a good opportunity for ESA participation with the lander and science instruments. NASA/ESA co-operation is on the agenda for the next OPAG meeting. * The mission does not assume big upgrades to the Deep Space Network. If the Next Generation DSN does come along, that's just gravy. * Despite the Europa focus, the mission appears to give at least part of a Galilleo II style tour with multiple flybys of the outer Galileans over 18 months. Only Io will have to wait.

The OPAG Europa working group is also expected to present further work at the next meeting in October. More details will emerge then. I think there is room for cautious optimism on this mission. While we won't be seeing a mission launch for at least another 7 years, the combined weight of the planetary science community does tend to get it's way in the long run. I think the momentum is finally starting to build.

Very nice work. (My saying this, by the way, has no connection whatsoever with your praising my work on the Great Pluto Probe War...) Absolutely the only error I can detect -- and I've long been obsessed with Europa exploration -- is that the original design for Europa Orbiter DID have a big radar array that in fact looked very much like this new one; it was just located at a different place on the craft.

And, yep, they seem determined now to add a very big HiRISE-type camera, in addition to the much smaller one they planned from the start. Not only are high-res shots of Europa important for understanding its surface processes; they're crucial for figuring out how to safely land spacecraft on what looks like a VERY rugged surface. While I'd love to see a small lander (if properly designed) added to this mission, however -- as would Jack Farmer -- it is very much up in the air whether they'll have the money to do so. (I'm currently planning a future article arguing that the best possible design by far for a small lander on this mission would be a penetrator rather than a surface lander.)

45% to 50% of the cost of the Europa orbiter mission is the cost of mission design and test.I would build 4 more space craft of identical design except for the mass set aside for the lander/probe.this mass would be used by a payload best suited for the target planet. targets? saturn(a cassini follow on) uranus and neptune. the 4th spacecraft would be insurence or a mission of oppertunity.

45% to 50% of the cost of the Europa orbiter mission is the cost of mission design and test.I would build 4 more space craft of identical design except for the mass set aside for the lander/probe.this mass would be used by a payload best suited for the target planet. targets? saturn(a cassini follow on) uranus and neptune. the 4th spacecraft would be insurence or a mission of oppertunity.

It might be neat to buffer one up and put a more appropriate instrument sweet on it, and send it on a tour like that of Galileo's later years (in other words, flying by Io repeatedly and occasionally other moons). With a Hirise likecamera, it could study Io's temporal activity, with closeup coverage every few weeks.

And, yep, they seem determined now to add a very big HiRISE-type camera, in addition to the much smaller one they planned from the start. Not only are high-res shots of Europa important for understanding its surface processes; they're crucial for figuring out how to safely land spacecraft on what looks like a VERY rugged surface. While I'd love to see a small lander (if properly designed) added to this mission, however -- as would Jack Farmer -- it is very much up in the air whether they'll have the money to do so. (I'm currently planning a future article arguing that the best possible design by far for a small lander on this mission would be a penetrator rather than a surface lander.)

The thing that worries me is that we are, by necessity, playing a game of Twenty Questions with Europa, and a big, battlestar-galactica craft asks a lot of questions at once, meaning that some of them may be mis-asked. (Like the fluid-probing instruments on Huygens.)

A big camera plus a possible lander could serve purposes, for sure, and if they came for free, who could complain? But look at how many missions we're using to pursue Mars exploration... Given that Europa is also going to take a lot of missions to crack (surely spread over a much longer span of time), a smaller scope might be called for.

Note that with a powerful camera, not much of the surface gets mapped: MOC on MGS will end up mapping only a few percent of Mars's surface (it was have been about 1.5%, IIRC, in the nominal mission). The kicker is, a Europa Orbiter won't live for a decade like MGS, but a month, so *very* little of Europa's surface will benefit from the camera's work. Granted, an attempt to image representative sites (both typical and the odd, atypical feature) should return a weighty fraction of the scientific knowledge that a comprehensive high-res mapping would, but the value of the camera still has to be weighed against that. I suppose the idea would be to produce *final* imaging of potential lander sites, and you have to do that sooner or later, so why not now? Well, the answer might come once the other instruments have had a look.

For a lander, that goes in spades. If we find something out from the hi-res camera, that could really affect lander design.

I think it'd be wiser to launch a probe with radar, a good-not-great camera, no lander, and have a quicker turnaround til the next mission. Europa's not going anywhere. Let's be methodical instead of extravagant.

The thing that worries me is that we are, by necessity, playing a game of Twenty Questions with Europa, and a big, battlestar-galactica craft asks a lot of questions at once, meaning that some of them may be mis-asked. (Like the fluid-probing instruments on Huygens.)

A big camera plus a possible lander could serve purposes, for sure, and if they came for free, who could complain? But look at how many missions we're using to pursue Mars exploration... Given that Europa is also going to take a lot of missions to crack (surely spread over a much longer span of time), a smaller scope might be called for.

Note that with a powerful camera, not much of the surface gets mapped: MOC on MGS will end up mapping only a few percent of Mars's surface (it was have been about 1.5%, IIRC, in the nominal mission). The kicker is, a Europa Orbiter won't live for a decade like MGS, but a month, so *very* little of Europa's surface will benefit from the camera's work. Granted, an attempt to image representative sites (both typical and the odd, atypical feature) should return a weighty fraction of the scientific knowledge that a comprehensive high-res mapping would, but the value of the camera still has to be weighed against that. I suppose the idea would be to produce *final* imaging of potential lander sites, and you have to do that sooner or later, so why not now? Well, the answer might come once the other instruments have had a look.

For a lander, that goes in spades. If we find something out from the hi-res camera, that could really affect lander design.

I think it'd be wiser to launch a probe with radar, a good-not-great camera, no lander, and have a quicker turnaround til the next mission. Europa's not going anywhere. Let's be methodical instead of extravagant.

Well, actually, in its low resolution channel, MOC has mapped the planet many times over at resolution of a few hundred meters. If the big camera comes with something like CTX, it could be quite useful. I also think that we are in a bit of a better positon with regard to a priori knowledge at Europa than at Titan. With a lander however, that is not the case. I don't think we know about the surface at a fine scale well enough for good site selection, although a relatively simple penetrometer wouldn't hurt - I just think it is early.

I think you have to balance the size and capability of the spacecraft against the permissible frequency of visits. Because of Europa's distance, and the large delta-v required to go into orbit, a sequential program like that for Mars is not going to be feasible. So there is more demand for the number of question-answer cycles to be kept to a minimum, even if that means more capable, and hence more expensive missions.

When it comes to any form of lander, I think anything complex will send mission risk and cost too high. But penetrometers may be vulnerable to being axed once the squeeze begins for spacecraft resources and funding, even if we are looking at 7 tonnes for the mission. The Decadal Survey *did* identify a Europa orbiter and lander as separate missions, after all. If the lander is an international contribution, that would make it more secure.

The HiRISE style camera is interesting. Certainly the 30 day prime mission is way too short a time to return the amount of data involved in mapping at that kind of resolution. Since the mission will have a wide angle camera for the global mapping, the question would be where to aim the big mirror. One aspect of the mission that would help is the many flybys and steady final approach to Europa before orbit insertion, which would give lots of opportunities for preliminary surveys. Also, if the mission carried a lot of onboard memory, then once in orbit thumnails could be sent, and then selected detail returned. But at that point the mission team would have to make up its mind *fast* (i.e. on a daily basis) on what was to come back in high resolution.

With a HiRISE style camera, you can do an enormous amount of high quality "raster" style mapping of moons from a distance. Either with a framing camera or a pushbroom sensor (I'd go with framing cause of low light levels), you couild build up low-distortion gigapixel mapping mosaics before the spacecraft moves a lot or the moon rotates a lot.

Somebody do the math and tell me for Galileo-type orbits, what resolution you get 6, 12, 24 and 48 hours from a flyby of the 3 ice moons, and what resolution <km/pixel> you get on Io.

Even a simple very small lander (a kilogram of instruments) would be useful for learning some things that we'll need to know for a more complex lander/rover. The Hi resolution camera won't be able to resolve things better than a few meters, and will be unable to actually test the surface composition.

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